After receiving a much-appreciated tip from another collector, I was able to bring home this 21CT664U set last night. It is missing the back, and the CRT is broken :tears: but the cabinet is in excellent condition, except for a drink stain and a couple of thin scratches on the top. The chassis is also nice and clean other than normal dust accumulation. The CRT "was" a 21AXP22A with the code 6048 on its connector, so I am guessing it was replaced after maybe five years of use. So, now I need to start a hunt for a good one.


http://70.36.238.5/ctc4b/IMG_2070small.jpg

http://70.36.238.5/ctc4b/IMG_2071small.jpg

http://70.36.238.5/ctc4b/IMG_2067small.jpg

http://70.36.238.5/ctc4b/IMG_2065small.jpg

Hey Chris, glad you got that! I got a tip on that yesterday, but I just couldn't see driving almost two hours to take a risk on something that was already out on the curb for free with no contact info. Noticed the ad was taken down fast... I had my fingers crossed it wasn't a tanker.... or worse, that person who makes the cat condos with the little dangling catnip mouse.

Way to go!

Nice find! Rare model.

-Steve D.

vary nice , too bad about the crt .

That cabinet looks to be in excellent condition. Nice save!

Beautiful set!

If you look in through the neck with a flashlight, you may find the gun assembly embedded into the shadow mask. Looks like it may have been launched forward when the neck snapped.

Ooh, nice! If you run across a stash of good 21AXP22s, let me know :)

Meanwhile, it looks like my adapted 21FJP22 will work, although I still haven't done any of the color setup, and have various other loose ends to tie up in my CTC-4 restoration.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

Probably did that while moving it to the curb.

Yah, it sucks to think it may have survived sitting all these years, and then got
broken just recently..... But anyway it is a nice looking set, keep yer fingers
crossed a picture tube will come from somewhere to you...... Good Luck...

Nice set

I hate that when you can see the after-effect of the gun assembly going thru the shadow mask.

Congratulations on a wonderful find!! Hope you can find a good 21AX, but if not, you can drop a 21CY, or a 21FB into the mask and set that up. Need to be sure that the anode connection doesn't arc to the yoke flange, otherwise, a relatively easy install.
Good Luck!!
Kevin

Thank you all for the nice comments, and for the suggestions about all-glass CRT substitution. The cabinet is in such nice condition, I kept looking at the back edges and elsewhere to see if it had been refinished-but it has not!

Regarding CRT substitution, the CTC4's yoke mount is interesting-it appears to be just about held in air by three metal rods that go to cabinet anchor points above and below the CRT near its front. That seems to me to potentially allow a glass CRT to be attached somehow to the front of the cabinet without needing to clamp the yoke onto the new CRT, which would be a big challenge otherwise. In fact, I also have a CTC-5 that may need a glass CRT as well, and I am going to look at similar mounting options for that set's yoke and CRT.

Glass tube conversion with pics

http://www.videokarma.org/showthread.php?t=253870

This article describes how I converted my CTC-4 to use a glass CRT:

http://antiqueradio.org/RCACTC-4ColorTelevision.htm

It's not very difficult.

The article isn't finished (nor is the restoration), so I haven't hooked it up to the rest of my website.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

Phil's comprehensive article on converting glass to metal CRT's is probably the way to go. Just for reference I thought I'd post a photo of my CTC-5 Wingate with the RCA metal to glass CRT conversion kit. When I purchased the Wingate many years ago it already had the conversion kit installed. The previous owner had converted the original 21AX to a 21GUP22. I personally have never seen one of these RCA factory conversion kits offered on Ebay or any other source. The kit has a large diameter metal ring with welded attachments at 11-1 o'clock & 5-7 o'clock positions that screw into the existing CTC-5 mask to hold the tube in place as seen in the photos.

-Steve D.

There is no good way to attach the CRT to the face plate of the set, and keep it easily revertable to original. Use Phill's method, and don't re-install the plastic shield...Also I have been able to get away with sandwiching plastic between the metal yoke mount and the HV button to prevent arcing instead of cutting the yoke ring...

Mstaton-

Thank you for posting that topic link, including your pictures. I know I had read it before, but I forgot it. That will be a big help.

Phil-
Thanks for posting the link to your CTC-4 page. I will read it as well.

Steve-

Thanks for posting your Wingate pictures with the RCA conversion kit, it is nice to see the "official" way it was done.

Tom-

I appreciate your notes and comments, I will look at Phil's notes too.

Some time ago, I picked up a used 21FJP22 along with its mounting hardware from a late-model RCA set (CTC-16 or 20) that my be part of what I can use, if I am lucky.

Phil, I read your description of installing an all-glass CRT in your CTC-4. You almost make it sound easy! I appreciate the detail that you put into your restoration description.

Part of the high cost of these early color sets were all the mounting and safety precautions needed. Once they went to glass tubes they were able to simplify to a degree the whole process. Still they found a way to get the job done.

I recently decided to proceed with the restoration of this CTC-4. It needed a bunch of tubes, the usual complete replacement of wax/paper and electrolytic capacitors, and a few resistors (oddly, all low-value ones that had drifted higher; none of the high-value ones tested bad). I connected it to my Sylvania test jig with a Variac and ammeter, and it had a raster on first power-up, woo hoo!

Initially, I got no signal through it, but with a few tests using a Sencore VA62, I found it had a bad video detector diode. That diode was already a replacement; since Phil Nelson also had a bad one in his CTC-4, I wonder if these sets are hard on those diodes. With the diode replaced, I get a pretty good signal through the RF input:

http://70.36.238.5/ctc4b/IMG_2990small.jpg

I started testing the UHF tuner using the VA62's output, which can be set to any channel up to 83, and found variations in performance among several 6AF4 UHF oscillator tubes. With a new RCA tube in place, UHF works well all the way to the top of the band, amazing to me for a consumer product sold in the mid-1950s.

Now for the bad news: I decided to go through the horizontal alignment procedure from the Sams Photofact folder, which includes adjusting the horizontal waveshaping and horizontal hold/frequency coils, followed by the HV and linearity adjustments. Well, I did the waveform adjustments and everything seemed fine, then as I got ready to connect my high-voltage meter, I saw a big, bright arc inside the HV box, almost like a fire had started in there! I pulled the plug quickly, and after a minute or two to let my heart calm down, I looked at the flyback and felt it; it was pretty warm, and most of the rubber "tire" coating had sagged down, exposing the top where the arc went to the top of the HV box:

http://70.36.238.5/ctc4b/IMG_3021small.jpg

http://70.36.238.5/ctc4b/IMG_3026small.jpg

(The sagging is to the left in the first picture, which was taken with the chassis flat; normally, it is vertical.) You can see where the wax has drooped low enough to completely expose part of the coil on the lower-left part of the "tire" in that picture.

Before I saw the wax damage, I powered up the chassis after everything cooled down, and as soon as the HV came up, the arcing started again, so I pulled the plug quickly of course.

Apparently, the "waveform adjustment" procedure, which has steps such as "turn the coil until the slug is all the way out, then bring it back" (putting the horizontal frequency way off for a few moments), caused the flyback to get warm enough to start melting the wax, and I did not think to watch it or otherwise be concerned, until the arcing happened. Aside from hating myself for messing up a part that was fine for 50+ years, my next step will be to remove the wax and inspect the arc spot for damage, to see if I can just apply a bit of corona dope there, then cover the whole tire with sensor-safe RTV silicone and hope for the best. If I remember right, the picture was still on the screen even when the arcing first happened, so I hope that means the flyback may still be OK once re-insulated. I tested its resistance from the H output tube cap to the HV rectifier tube cap, and got 490+ ohms (versus 510 ohms on the Sams schematic), so it is not open but could that be a sign of some shorted turns?

The remainder of the horizontal adjustment procedure is to set the high voltage, and adjust the horizontal linearity for minimum current to the yoke. So, there is no step to check the H output cathode current, but should I measure it anyway, and possibly re-adjust the waveshaping coil while monitoring that current (as well as my oscilloscope), if in fact the chassis seems to work again OK once finish the flyback repair?

This is why I always use the utmost of caution when working around the horizontal section of these sets, if you fry the flyback it's impossible to find another one. That means you'll probably end up using a completely different flyback if the one you have can't be repaired.

It's not coated in rubber BTW, it's wax. I just repaired a flyback from another set by replacing the wax, see my thread on the Gilfillan prototype. Use extreme caution around the tiny wires of the HV winding, too much heat can melt them. I used a soldering iron to carefully melt the old wax off, then used a silicone mold to cast the new section from a negative of what was still there of the old section. Electronics safe RTV works too, I was trying to go for originality because my flyback has a rectifier string around the outside of the tire. You could also put a piece of phenolic over the metal section of the cage it arced to, that would help keep any stray HV to a minimum.

Good luck, it's not an easy job.

I would find it strange for the fb to be destroyed from a short use with one of the
shape coil not properly adjusted. I would put more stock into either there is
something wrong with the part, as your resistance readings possibly indicate, or,
too much current through it.... Was Horiz Output Tube cathode current within spec..?
Also, if the resistance was off, I would imagine cathode current would also be off....

If you can pull off a fix for the arcing, I would check cathode current, and the grid
voltage, too far positive and it's going to gobble up too much current. And look
at the shape of it signal, no dc leaking in either....

So is that flyback completely unavailable....?

Good Luck !

.

Only source is other chassis, no one made 'aftermarket' ones.

Adjust the horizontal linearity for minimum H Out Tube cathode current....If that is miss-adjusted you could be cooking the flyback.

My CTC7 had the same problem, it had a hole burned through the rubber insulation where it arced to the metal cage, luckily I found it before it was fired up.

I was able to remove the covering, clean the burnt area and recoat it with some low acid RTV and it works fine. As long as it hasn't burned through the wires it should work on yours.
A dab of High Dielectric Epoxy paint on the area before coating it might not hurt either, I didn't do that but mine wasn't burned much at all other than the cover.

Adjust the horizontal linearity for minimum H Out Tube cathode current....If that is miss-adjusted you could be cooking the flyback.
..as well as the H. output tube.

Great Find!!!!!!!! Sad about the necked CRT. But fear not, one will turn up at some point. I guess I am something of a purist. My feeling is that the most important reason for having a CTC4 is to have the 21AXP22 crt in a functioning set. That original crt is in large part, what makes the set rather historic. The 21CT55 was the only earlier set to use the 21AX. The first mass use of the 21AX occurs in the CTC4 series. (some clones of limited production numbers were sold by other manufacturers in very small quantities).

The original CTC4 flybacks are prone to failure. ManyMany were replaced with an improved version. The original flyback is part #110409 (drawing #1106237-1). The improved replacement flyback is 101959-A and also involves a number of circuit changes to the chassis which RCA made to accommodate the new flyback design. According to the drawing # stamped on your flyback (1106237-2) you have a replacement flyback in your set. There should also be a paper hang tag inside your set explaining that you have a replacement version flyback and that the chassis has circuit changes to accommodate that replacement flyback.

When you start to work on the chassis you are going to find a number of circuit changes which will not match the original RCA schematic. Those changes are on a paper hang tag that should be attached to your set (provided some numb skull didn't pull the tag off and discard it.) (there are instructions on the tag that tell the service tech to attach the tag to the inside of the set after installation of the replacement flyback)

I believe someone here on VK (possibly Nick?) posted a photo of the hang tag at some time in the past which has all the circuit changes that have been made to your chassis.

You have a Wonderful old color set there, and it is quite an uncommon model. According to my RCA service bulletin, the Gainsborough came in Walnut or Maple finish. Which is yours? Hard to tell in the photos. Looks like Walnut to me.

My Cheltenham is in natural Birch finish But over the years the color has mellowed and it looks more like Pine now. I just completed the re-finishing of my CTC4 Cheltenham cabinet, and I am about to start the re-cap of the chassis.

I think my CTC-4 thread has pics of the hang tag in it, if not I can email them to anyone in need.

Thank you all for the helpful information and comments.

Nick, I may put a piece of phenolic on the top surface of the HV box if/when I get this set going OK again; does "fish paper" accomplish the same thing, if I find some of that instead?

Username1, Tom, and Old Coot, I will go ahead and put a current meter on the H output cathode when I am ready to fire this set up again; also, I will run the Variac low to start, and observe the results of adjustments I make. The adjustment procedure for a CTC-4 does not specifically address the cathode current, but I will do so anyway.

Eric, my thought was to use some corona dope on any suspicious windings in the arc area once I have the wax carefully removed (and before I coat it with the silicone). Do you think it is a viable alternative to epoxy paint?

Bob, this set has a CTC-4B chassis, and seems to match the "late production" schematic diagram in the RCA 1955-1966 Field Service Guide. I thought I saw an RCA note that said all Gainsboroughs were "late production" models and were built with the changes in place; there is a list of them in the Guide that I will look at and compare with my set. There is no hang tag or any other service information in the cabinet or chassis. This is a maple-cabinet set, I think; it is a much lighter color than any walnut wood I have seen. Indeed, I would love to put a 21AXP22 back into it one day. The broken CRT was a 21AXP22A with the date code 6048, and from various tube date codes, this set was likely in regular use to perhaps 1966 or so.

We're talking 24kv+, I don't think paper is gonna cut it. You need something thick with high dielectric strength like phenolic, nylon, plastic, ect. FWIW, once you fix the obviously high horizontal current situation I doubt you'll have the issue crop up again. It happened because the insulating wax melted away, allowing the HV to find the path of least resistance.

I was gonna say the same thing re. fishpaper. It's useful in lower-voltage applications, but I sure wouldn't use it here.

I used some of this 90KV-rated cambric tape when transplanting a glass CRT into my CTC-4. I still have much of the 20-yard roll left. Send me a PM and I'll mail you some to insulate that spot where the arcing occurred.

http://antiqueradio.org/art/RCACTC-4VarnishedCambricInsulatingTape.jpg

I would reserve judgment on the flyback until you can remove the wax and carefully inspect the winding.

Apart from this little incident, if you're getting a good picture from RF input, that sounds like great progress.

Regards,

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

Chris,

Just goes to show you learn something new every day. I had always wondered about the 3 different CTC4 chassis. I did some investigating and in one of my RCA service bulletins I found a chart defining the 3 different chassis. The 21ct660u and 661u in the two low end Seville and Haviland models, used the early production CTC4 chassis. The 21ct662u Director, used the CTC4A. The 21ct663u Cheltenham and 21ct664u Gainsborough used the CTC4B.

There are a fair number of circuit differences, mostly in the color circuits, between the CTC4 and CTC4a. I don't have a CTC4B RCA schematic so I am not sure where the circuit differences lie in the CTC4B version.

Unfortunately I got my Cheltenham without a chassis. It was just a gutted cabinet. I have a CTC4 and CTC4a chassis to work with but I do not have a "B" version. And what is worse, the CTC4A I have has been canabilized so I am not certain which direction I will proceed. I am leaning toward rebuilding the "A" chassis but that will entail finding a pile of spare parts that are missing. Clearly the CTC4 early version is not correct for a Cheltenham.

Nick and Old Coot, thank youfor the comments regarding fish paper. I just had no idea of its properties.

Phil, thank you for the tape offer. I will get in contact with you once I evaluate the flyback. As Nick said, reasonably it should not come up again; it would be more of a "just in case" plan but still maybe worthwhile. And, I will likely want to use some of that kind of tape for the CRT HV connector as well, once I am ready to install the chassis into the cabinet.

Bob, I wish you luck with that Cheltenham restoration (maybe, reconstruction is more like it), I know you are a person who can definitely succeed with the project. If I can help with identifying details of the CTC-4B chassis, I will be happy to do so. At the least, I could take and forward lots of high-resolution pictures from various places and angles. My chassis had very little work done other than tubes over its likely ten years in use; I only identified a couple of obviously-replaced capacitors plus the video detector.

Today, I had both a funeral and a banquet to attend, so I only have had time to get online a bit, but tomorrow I hope to evaluate the flyback and post an update.

Here is the location of the arcing, after cleaning most of the wax from the flyback HV coil.

http://70.36.238.5/ctc4b/IMG_3036cropped.jpg

The view in this picture is about 5/8 inch wide, with the darkened area being just over 1/4 inch long. My main concern would be whether the dark parts are now carbon or something else that I should attempt to remove, before applying some corona dope (or other substance such as HV-rated epoxy as Eric mentioned?), followed by the silicone.

Also, I doubt that I could remove "all" of the wax on the coil, especially after seeing discussions of vacuum-impregnating flyback coils. I would be concerned that the materials or methods to do so could be risky themselves. Is some remaining wax likely to doom the silicone coating integrity as a whole? I appreciate any suggestions here.

Great Picture ! If it were me, I would use a tooth brush, try and clean all that burnt
looking stuff off. Try and determine if there is a lot of damage to the insulation of
each wire.... You may need to treat that, varnish ?

I read the piece on the hot wax and the vacuum pump, really good ! It was mostly to
promote boiling off of moisture. Lower the atmospheric pressure and water boils
off at a lower temperature.... Just as they use a 12 Lb cap on your car radiator, it
increases the boiling point of the coolant.

You have to be sure after you coat it with whatever your going to do, it has no air
pockets, it can over time promote oxidation and corrode and open up a wire....

But it looks to me the wire is in good shape ! clean it coat it,
patch that tire, and back on the road !

:)

.

Thanks. I am carefully using a pin point to remove what I can. Even with a magnifying-lens lamp, I cannot get as good a view as the camera can, but I will try more pictures after some cleaning work.

I'd probably try to clean that spot a bit better. There is no need to Vacuum impregnate the fly with wax, and I think putting silicon on the fly is overkill. What I'd do is heat the portion of the wax tire that fell away (probably with a hair dryer) till it became soft enough to smoosh back on to the fly, do that and mold it back into the shape it should be in. If that proved impossible I'd try making a mold or form around the fly to pour molten wax in to cast a new wax tire.

The wax can always be remade if it fails, but silicone is going to be hard to remove if bad.

That char contains a lot of carbon which of course is conducive to shorted turns.
After cleaning as much as humanly possible, I would try this before applying any sealant:
With the 3A3 removed, power up the chassis (preferably with a variac) and with the room darkened, look for any sign of incandescence in the cleaned area. Any specks of incandescence would indicate leakage paths between turns.
----
(Edit) Why do I think shorted turns would incandesce in this instance? Well, since a couple of turns of wire generate enuff juice to light the 3A3 filament, a single turn ought to light up a speck of carbon white hot.)

And if there's no incandescing, see if any heating can be detected in the cleaned area. An IR camera might come in handy. If there's no heating detectable it's probably good to go.

Here is another picture. Any cleaning better that this would probably require some kind of solvent, which would likely damage the good insulation nearby as well. I am planning to coat this area with corona dope before I apply power in any case.

http://70.36.238.5/ctc4b/IMG_3047cropped.jpg

Tom, the original wax is not feasible to re-use, as far as I could imagine doing. My concern with trying wax again is that if anything happens in the future such as a short at the same spot, another wax meltdown would likely destroy the part and maybe much worse. You have a good point about wax being more easily re-done/reversed, though.

Old Coot, without the HV rectifier, am I still looking for/expecting the full high voltage to develop in the flyback coil, at least if I get the variac anywhere near 110+ volts? If I am, then I would need to plan for and insulate against new arcing, I would expect, since the entire coil is now uncoated.

Another alternative I will consider is to remove part of the coil winding to a point below/before the damaged area. Tom Albrecht did this successfully on a flyback that had a shorted turn near its outside layer.

Great picture, you can clearly see the portion that got zapped. What you're looking at here is the outer turns of the step up ring, which consists of very fine copper wire wrapped with a single layer of even smaller fibers. The burned portion has had its covering exposed, which is why it looks different from the adjacent turns. I would very carefully attempt to remove any remaining burned stuff with a microscope and dental tools, then apply melted wax with a small brush till they are again covered. You could then install it with the cage removed to see if it will work, then take it back out and finish building the tire back up. This is going to take time to get right, so take it one step at a time and don't rush. Those wires are close to the size of a human hair, doesn't take much to break them.

Thanks for the detailed notes, Nick. I will see if I can get more of the burned material off the wires before any coating or tests.

friggin cell phones, sorry I was riding in a car when I typed that. Cleaned it up.

I've sprayed Flys and HV coils with this stuff, it's good for 2100 volts per mil of thickness, that doesn't sound like much but you don't need to insulate for the full 20kv, just the difference between one winding and the next.

I believe there are higher voltage versions of this stuff also.

Nick, I understood OK even before the editing, thanks. We must all be getting used to cell phone messages.

Eric, thanks for the link. That HV rating (2100 volts/mil) matches my MG Chemicals corona dope, so I should be OK with it too, I would think. I am applying a few coats of it in the area where the arc occurred now, after further cleaning of debris there.

Tomorrow, I am going to pick up a piece of glass-epoxy board for the top of the HV box (copper plated on one side, which can go toward the box's metal surface). It is rated for 40KV "dielectric breakdown" strength, which I hope is the property I should be looking at:

http://www.alliedelec.com/images/products/datasheets/bm/MG_CHEMICALS/70125831.pdf

An opportunity exists for you to do the best job possible - but it's more work. Since that
flyback is one of those non-obtainable types.... It may just be the best idea to combine
some of the aspects of the best ideas people have posted - into the best possible repair...

Since the damaged area is at the end if the most outer windings, you could unwind it to
the trouble spot, then aggressively clean off the burnt crud, even if you need to use
something like sand paper.... Then coat the bare wire with the 2100 V. spray - maybe
even several coats.... Then if the wire has a wax coating on it as if it were done before
the wire was wound onto the transformer, then you can redo that too....

While the spray protects to 2100 V. it will easily protect the low potential from one
winding to the next, but your arc, and the next most probable arcing will take place
from the highest potential to the lowest, like the metal cage, just like last time.
So it would be smartest to take the most protective precautions available to you.

You will most likely need to replace the tire, or repair it at the burn spot since it is the
item that was in place between the area of highest potential, to that of lowest potential.....
Insulating the inside of the metal cage is also a great idea.....

Good Luck !

.

you could unwind it to
the trouble spot, then aggressively clean off the burnt crud, even if you need to use
something like sand paper....
.
If he should go the unwinding route, there's a neat method of cleaning hair-fine wire that avoids mechanical abrading. Scroll down to post# 26 http://videokarma.org/showthread.php?t=257295&page=2&highlight=aspirin

Username1, thanks for the repair ideas. I am going to replace the tire, likely with sensor-safe silicone, after testing it; it was a mixture of crystallized as well as more-pliable wax when I removed its pieces. I have some high-voltage putty that I can coat the coil perimeter with for the tests, but I want something more durable in place long term.

Old Coot, thank you for that link. I will read through the discussion.

Success, tentatively! :)

I let the multiple coats of corona dope on the arc location dry overnight, installed the insulating board in the top of the HV box, and put a rope of high-voltage putty around the flyback coil:

http://70.36.238.5/ctc4b/IMG_3053small.jpg

http://70.36.238.5/ctc4b/IMG_3059small.jpg

In the picture of the insulator, wires are visible under each end of the board. It is a copper-clad board on one side, so I soldered those wires on to ground the board under nearby screws; they will also hold the board in case the adhesive holding it fails. In the flyback picture, the arc location is at the "3 o'clock" position, to the right in the picture. That was all the putty I had, or I would have used more. I plan to replace it with silicone permanently in any case.

Before I applied any power, I did a run-through of the whole process of possible actions and adjustments, and I found that my horizontal linearity coil (also called horizontal tuning, and horizontal efficiency in later sets) was frozen. I had to remove it and apply heat from a heat gun, but it freed up nicely. (Thanks to Tom, Electronic M, for that tip from his own CTC-4 coil, in another discussion I found.)

With the coil reinstalled, I put my Simpson 260 inline with the H output tube's cathode, put my HV meter on the anode wire to the test jig, connected the Variac, and increased the voltage in moderate steps. At 80-90 volts, the set slowly came to life, and I watched the HV go to 20kV without any problems. The H output current was low, perhaps 120mA on the meter even with the Variac at perhaps 95-100V, and I did a brief adjustment of the H linearity coil without seeing a big change. At this point, the set had been on for 4-5 minutes, so I shut it down to check the flyback temperature. It was totally cool on the perimeter of the HV coil, including at the arc location, with a bit of warmth in the "spoke" area of the "tire".

I think I will replace the Simpson 260 with a new meter (still keeping the 0.1uF capacitor across the test leads) and try another test, also maybe running the AC voltage a bit higher, before saying the test was good and preparing to coat the flyback coil with silicone.

One question I have: Should the HV adjustment control give me a linear range of voltage, even with the AC input low, or does the HV regulator circuit function more as a limiter above a certain voltage? Are there any dangers to setting it too low while testing its function, if in fact I can? The HV got to 22-23kV already even with the AC input at perhaps 100V or less. I appreciate any advice here.

Glad my CTC-4 thread helped....Makes me think I should get back to that set.

The regulator is a top end limiter, but IIRC it is referenced to boost or a similar fly derived source so it will dip with line voltage somewhat. There is not a lot of problems with the reg being low (for testing), but that has some dependencies.....If the set wants to produce 40kV with the reg cap off and the reg is dropping it to say 10-17kV the reg load on the fly may heat it up and be detrimental over moderate to extended run times... If it's on for under 10 minutes it probably will be okay.

I adjusted my oscillator to perfection with a scope before installing the H output tube, and the adjustments necessary with that tube in, like linearity current dipping, I tried to be totally ready for prior to switch on and perform as soon as it seemed to settle from initial warm up....Perhaps I was over cautious but I never did any harm or had any troubles with the horizontal/HV system in doing it that way.

Chris,
Just a question purely of side interest: Do you notice any difference in the 260's current reading with and without the .1 cap across it?

I've never found it necessary, guess someone found a problem at some point that it cured. Maybe today's modern meters are better? Dunno.

I always used a 260 for K current and never thought of putting a cap across it, since it's functionally a dead shunt. Be interesting to see if there's any difference with/without the cap.

Tom, thanks for the notes about the horizontal circuits. I would never have thought to adjust the waveform with the H output not installed, I guess I am too much of a "by the book" person normally.

Old Coot, I used the capacitor because (again) "the books" typically say to do that. The official CTC-4 procedure does not even address the H output cathode current, but rather the screen grid and/or damper current. I will try another test with and without the capacitor and let you know if I see a difference. My other test will be with a DMM, and maybe those are the meters that need such a capacitor across them. (That test will be because I am not 100% sure of my 260's performance.)

Nick and others with better technical background than I have; please chime in here and discuss "CRT LOAD"

I looked up the 2nd anode load for a 21AXP22. It said 500 microamps per gun maximum or 1.5ma total 2nd anode load. (I am assuming full brightness). At 25KV that converts to 37.5 watts (if I did my math correctly 25,000v x .0015A = 37.5 watts) That is certainly enough to warm up the flyback, but I would think the CTC4flyback might be able to dissipate that much heat safely.

It is my impression, from discussions concerning blooming under heavy video demand in bright scenes, that a very heavy demand is placed on the flyback during these situations. 21CT55 uses a modified ct100 chassis and the horizontal drive section of the CTC2B chassis is woefully inadequate to provide enough current to the flyback during these bright situations. As a result you can get picture blooming and fade out on the crt.

That leads me to believe that CRT places a high demand for 2nd anode current during these bright situations. Therefore can I assume because Chris is using a small crt test jig, that we are never going to see real world demand on this HOT until he hooks up the 21AX and it's associated deflection yoke?

The point I am getting at is this; 120ma of cathode current is no where near the kind of load that will be drawn by the cathode of the horizontal output tube during actual use with the 21AX. Therefore any cathode current readings or temperature of the windings need to be done under full load conditions. When full current is demanded during bright scenes on a 21AX with 22Kv or better, then and only then will we be able to determine if the flyback is going to over heat. And I think from dead cold to full temp under normal loading conditions will probably take at least 30 minutes to achieve.

It is also my gut feeling that because current and voltage are inverse to each other, that a low voltage at the 2nd anode, will cause a rise in current. It is the rise in current that will causes the flyback to heat up. Think of an arc welder. More current (amps)=more heat.

The thing that causes a wire to heat up is the current being drawn through it. Seems pretty basic to me. But I would guess there may be other factors involved.

As for the 6BK4, that sucker (essentially a high voltage triode) is designed to limit the upper end of the voltage range by shunting the 2nd anode when the voltage gets to high. So that leads to the next issue. If you have a flyback that is producing (for example) a consistent 30KV and the 6BK4 is set to start regulating at 25KV, isn't the 6BK4 placing unnecessary continuous load on the flyback? Wouldn't you be better off bringing down the output voltage of the flyback so the 6BK4 wasn't loading the flyback output. Sort of like putting your foot on the accelerator of your car and regulating your speed by applying the brakes. You would be better off to ease back on the accelerator instead.

Perhaps Chris' flyback wax melted because it was being driven harder than it needed to be, and the shunt regulator was constantly sucking up the excess and loading the flyback when it produced too much HV.

These are just my gut feelings (and rambling thoughts about a subject I would like to understand better), and to those members that understand this stuff better than I do, please chime in and lets have some good discussion on the cause and effect of real world current demand on the flyback from a crt that is being driven hard, and the effect of the 6BK4 on a flyback that is producing higher voltage than is required. The better we understand the causes and effects that damage our flybacks (mostly pure unobtanium) the better we can protect these irreplaceable components.

Inquiring minds need to know

AS an aside comment; I have a CTC4 chassis that I am working with for the Cheltenham. I turned the chassis over and what did I find? I found the HV doorknob cap with most of it's potting wax compound melted out of the housing. Obviously, the doorknob is leaky and producing a lot of heat as it draws current. This is just another place where unnecessary LOAD is being placed upon the flyback. After the ctc4 series these HV doorknobs were eliminated. I imagine the doorknobs were problematic. My 21CT55 had a bad doorknob and it wiped out the flyback. The doorknob should not have any DC leakage. It's job is to smooth the ripple on the 2nd anode.

On the CTC5 series, all that was used was the conductive dag coating on the exterior of the plastic crt shroud which acted as a filter capacitor between the metal bell of the 21AX and ground. When the 21CY glass tube came out, dag coating on the tube became the filter cap.

So if you have a CTC4 or a 21CT55, make sure your doorknobs are not leaky. I would think it can be a source of excess load on your flyback.

I looked up the 2nd anode load for a 21AXP22. It said 500 microamps per gun maximum or 1.5ma total 2nd anode load. (I am assuming full brightness). At 25KV that converts to 37.5 watts (if I did my math correctly 25,000v x .0015A = 37.5 watts) That is certainly enough to warm up the flyback, but I would think the CTC4flyback might be able to dissipate that much heat safely.


500ua per gun is probably a bit overly optimistic; not that the tube couldn't handle it as it certainly would be bright with that much available current, just that I seriously doubt any flyback ever made for an early color TV could produce it. My Director gets pretty close, it has the strongest horizontal section of all my sets and a bright picture even on a so-so CRT.

That doesn't mean the flyback necessarily has to get hot in the process though, if everything is tuned the way it should be it's a pretty efficient magnetic coupling device. The horizontal output tube has to bear the full load of anything downstream of it, so that means yoke, HV, other circuits like AGC and gating pulses, AND the rectifier filaments. If the yoke has leakage or some other problem for example you might not necessarily see it on the screen, but it's a load on the horizontal circuit nonetheless. Same goes for HV leakage paths to ground and surrounding circuits, dusty/dirty HV cages and old HV wiring all contribute to losses.

These are all reasons why each restoration I do begins in the HV cage and horizontal circuit.

It is my impression, from discussions concerning blooming under heavy video demand in bright scenes, that a very heavy demand is placed on the flyback during these situations. 21CT55 uses a modified ct100 chassis and the horizontal drive section of the CTC2B chassis is woefully inadequate to provide enough current to the flyback during these bright situations. As a result you can get picture blooming and fade out on the crt.


Same processing circuits, much different HV setup with better flyback and larger horizontal output tube. They made a compromise when they designed each set, though if you ask me they made a mistake with the CTC-5 not a compromise.

As for the 6BK4, that sucker is designed to limit the upper end of the voltage range by shunting the 2nd anode when the voltage gets to high. So that leads to the next issue. If you have a flyback that is producing (for example) a consistent 30KV and the 6BK4 is set to start regulating at 25KV, isn't the 6BK4 placing unnecessary continuous load on the flyback?

It usually doesn't take much regulator current to bring it back to earth from a free wheeling situation (like reading the anode with the 6BK4 out of circuit), but anything lower than a regulated 22kv or so usually strains the tube and it starts red plating. The Admiral I just picked up was set to 18kv, and let me tell you there were all sorts of interesting colors coming from the plate of that 6BK4. Probably got my annual dose of x-rays. :D




Wouldn't you be better off bringing down the output voltage of the flyback so the 6BK4 wasn't loading the flyback output. Sort of like putting your foot on the accelerator of your car and regulating your speed by applying the brakes. You would be better off to ease back on the accelerator instead.

Unfortunately it's not that simple, if you reduce the power produced you also reduce headroom. The HV winding makes what it makes as a function of how much power is put into the flyback primary, so it would be a much better idea to limit the power going INTO the flyback instead- AKA a pulse regulator as some later Zenith sets did. By limiting the amplitude of the pulse on the primary side of the flyback, it only makes what it needs instead of over producing and being dragged back down by a 6BK4. The RCA prototype does the same thing, with a 6BQ6 pulse regulator in parallel with the 6CD6 output tube. I often wonder why many more sets didn't use this regulation scheme, I'm willing to bet there'd be a lot less burned out flybacks if they had.

FWIW, most early color TV's don't make enough HV to be dangerous. I run my Wingate with the 6BK4 disconnected all the time, because even then it struggles to make 22kv on a good day with a tailwind. I wouldn't try it with anything earlier though, the first ones were absolutely lethal with HV production. Whatever you do, do not allow anode voltage to rise past the upper limit of what the CRT can handle.

Think of the CRT and the shunt regulator as two variable loads on the flyback. When the CRT has a bright scene, it becomes a greater load while the shunt regulator becomes a lessor load. Conversely, a dark scene causes the CRT to become a lessor load while the shunt regulator becomes a greater load. The whole idea here is to present a more or less constant load on the flyback and the circuitry upstream of it. The real question is whether the designers specified the flyback so that it can withstand this combined load (CRT plus shunt regulator) on a continuous basis without having too much of a temperature rise.

I always liked the Zenith pulse regulator although it wasn't as well regulated as a shunt regulator. I never had to replace a Zenith flyback with a pulse HV regulator.

Think of the CRT and the shunt regulator as two variable loads on the flyback. When the CRT has a bright scene, it becomes a greater load while the shunt regulator becomes a lessor load. Conversely, a dark scene causes the CRT to become a lessor load while the shunt regulator becomes a greater load. The whole idea here is to present a more or less constant load on the flyback and the circuitry upstream of it. The real question is whether the designers specified the flyback so that it can withstand this combined load (CRT plus shunt regulator) on a continuous basis without having too much of a temperature rise.

That is a good explanation (and a good part of the point I was going to make before reading it). I'd like to add that the regulator is also an "emergency limiter"....if say the CRT heater goes out for some reason without the regulator in place there is no load on the HV winding of the flyback, and it then can easily shoot up to 30-40kV in a CTC-4....Once it hits that range the factory insulation on and INSIDE the HV winding is near to beyond it's design rating, and prolonged operation under those conditions is likely to cause external and INTERNAL arc over and all the damage that goes along with that.

Most late 60's sets with a pulse regulator also get their focus from a resistor divider fed from the CRT's HV lead, and probably also serve as an emergency minimum HV load.

Chris, one thing that now comes to mind is that you are calibrating the horizontal and HV system with that set on a test jig (right?), the jig likely is not providing the exact yoke (and, as mentioned, CRT) characteristics the chassis will see with it's own CRT and yoke that are in the cabinet. I doubt all those adjustments will still be correct once the chassis is back in it's cabinet, and I'd be inclined to redo them when the chassis is ready to be mounted. My cabinet acted as it's own test jig when I was servicing my set (cables have enough reach to allow the back of the chassis to face the cabinet and reach), and upon reflection I'm glad I did it that way.

Good discussion guys! Now lets have some input on your theories as to the causes of flyback overheating in sets that have HV that is within acceptable limits.

It has been my experience that a transformer does not overheat unless it has an excessive load placed on it, or the transformer has shorted windings.

What do you fellows think? Are you going to point more to a defective flyback with a short or did something place an excessive load and that caused overheating and then a resultant short.

IF it is excessive loading, what was the most likely cause?

I appreciate this discussion of flyback and HV circuits, it is good to learn from others with more knowledge than I have with these early sets.

Chris, one thing that now comes to mind is that you are calibrating the horizontal and HV system with that set on a test jig (right?)
Yes, I am using a jig, and you made some good points, thanks. I will indeed re-check everything once I move the chassis to the cabinet.

Since I had the overheating/meltdown/arcing with the set powered on for a relatively short time (20 minutes or so perhaps) compared to having it run an hour or more several times previously without a problem, I suspect that I caused the overheating by running the horizontal circuits off-frequency too long or too far, as I did the waveshaping adjustments.

I did a new set of tests today, including running the chassis longer and at 110-115 volts, with no problems seen. After being powered on for over 20 minutes, the flyback was just low to moderately warm, certainly not warm enough to cause wax to melt off if it was present, and no warmer at the arcing point than elsewhere, so it seems to have survived. :thmbsp::D

Bob, I am not sure why I was seeing that 120mA H output cathode current reading, and in fact today's reading was 80mA for a few minutes before going up to closer to 200mA while I was looking elsewhere during testing. I have to suspect my Simpson meter (maybe dirty contacts; I have never used its current function since restoring and testing it a year or more ago), since there was no change at all in the appearance of the picture before and after the displayed current change. I checked the current with and without a 0.1uF/630V capacitor across the meter connections, and I also tried my Fluke DMM with and without the capacitor. Neither meter showed any difference without the capacitor, so I will say it is not needed here as some of you already said was likely. The meter readings (all of this was after the Simpson readings "jumped", and they stayed consistent once they did that) were: Simpson ~195mA at ~108V line voltage (from the Variac) and ~205mA at ~115V, Fluke 188mA at ~108V and 198mA at ~115V. I did verify that the H linearity coil was set for lowest current (well, as soon as it went even a couple of mA higher, I stopped and went back, in both directions; I had already adjusted it for a bit lower current yesterday).

I will plan to do current readings again after I put the chassis in the cabinet, too.

Bob: You are ignoring door number 3: The makers designed the set so the flyback would run hot, and not have sufficient power headroom or cooling provisions to do otherwise....Many RCA designs did this or were on the edge to the point that non-optimal conditions would push them over (though IMHO the CTC-4 was one of their better H deflection/HV system designs).

Chris: May I suggest a procedure to select operating voltage? When I was working on my CTC-4 I set the line voltage such that the highest B+ rail directly derived from the power transformer matched the spec in the schematic.

Another trick using the variac is to put your AC voltmeter on the heater supply (preferably right where it comes out of the xfmr), and set the variac for exactly 6.3 VAC, then see what primary voltage is.

Thanks for the Variac suggestions. I will try that, both ways. One day, I want to make a few line-bucking transformer setups for my antique TV sets.

Congradulations. Looks like you dodged the bullet since the fly is running cool with no localized heating at the damage spot, and the K current is OK.

This should be an object lesson on why one should never, ever leave any vintage TV, BW or color, running unattended, even for a moment.

Good discussion guys! Now lets have some input on your theories as to the causes of flyback overheating in sets that have HV that is within acceptable limits. It has been my experience that a transformer does not overheat unless it has an excessive load placed on it, or the transformer has shorted windings. What do you fellows think? Are you going to point more to a defective flyback with a short or did something place an excessive load and that caused overheating and then a resultant short. IF it is excessive loading, what was the most likely cause?

So you're asking the chicken/egg question? lol

The simple answer is we're working with 60-70 year old electronics at this point and it's a miracle any of it works anymore, but beyond that these are the culprits I've personally run into.

1. Running sets with higher line input voltage than called for in the service information. All the sets I've ever worked on were specified to accept 115-117VAC input from the wall, and the reality is these days we can have anywhere from 115 to 130VAC at the wall plug. There's not much regulation going on with the lines coming from the utilities, so during summer when everyone is running the AC it's usually on the low side and right now when it's cold at night time and demand is low it can get pretty high. This has unintended consequences with the horizontal circuits in a set but people don't think about it, because it doesn't affect anything else in their home because we now have switching supplies that account for it in the rest of our appliances.

Because you have a linear power supply with step up transformer in old TV's, the line drifting high causes the B+ supply to drift up as well. Only a few volts difference on the primary can cause 50+ volts extra to show up at the secondary, which then gets passed directly to the horizontal section and flyback transformer. Those extra volts will also cause a rise in horizontal cathode current all else being equal (the primary resistance of the flyback hasn't changed, so check ohm's law!), which means the flyback is being strained and it's natural to expect it to run hotter as a result.

Because of this, I refuse to run any of my sets without an autotransformer or variac and verifying AC input voltage.

2. Not having the horizontal section aligned correctly. This could be anything from a wave shaping coil set wrong to the HV regulator tube drawing excessive amounts of current, it all falls under the same banner. Some are content to simply go through the procedure in Sams and assume it's good, meanwhile it's running away with excessive current draw and next thing you know it melts down for seemingly no reason. I don't stop till I can put a number on HOT current, which I then annotate in the schematic as an accepted norm. This way if I ever mess with it again, I know what it was last time and can watch for changes.

3. Moisture. This one we can't do anything about so far as I know, but when sets sit all the components will be absorbing moisture from the environment. If said environment happens to be a damp/wet basement, watch out. It can infiltrate the windings of the flyback and cause the copper wires to corrode leading to shorts, intermittant pops, overheating, incorrect resistance readings, nearly anything you can think of within reason. I know shango dunnked a fly in hot wax and boiled out the moisture, but for the vast majority of us this will not be an option. Best thing you can do is actually run the set, because the heat from the flyback will drive the moisture out naturally if given enough time.

4. Failed CRT. Sometimes despite your best efforts a fly gets taken out by an arcing CRT, and you can't do anything about it. Sometimes parts fail, just the way it is. Doesn't have to be a reason which always sucks, but like I said 70 year old TV's...